Hydroxyproline, an amino acid found almost exclusively in collagen, results from the enzymatic conversion of proline in peptide linkage rather that incorporation of this amino acid directly in growing nascent collagen peptides. Although prolyl hydroxylase the enzyme catalyzing this reaction had been thought to be a soluble cytoplasmic enzyme for over a decade recent evidence has indicated the particulate nature of this enzyme in a variety of tissues. We propose to localize the site of the hydroxylation of peptide bound proline during collagen biosynthesis in lung tissue by characterizing the particle containing the enzyme involved in prolyl hydroxylation. Furthermore we will determine if the substrate for prolyl hydroxylase is located in this particle. We will also determine the relationship between particulate enzyme activity and the rate of peptidyl proline hydroxylation during collagen synthesis. The following questions will be answered by these studies: 1) Is protocollagen hydroxylated at a site in the cell where particulate enzyme is found? 2) What is the nature of the particle containing enzyme activity? 3) Is the rate of prolyl hydroxylation of tissues proportional to the activity of particulate enzyme? 4) Are particles containing prolyl hydroxylase able to function in an in vitro protein synthesizing systems?